Description: If there is a vertex
being incident with an edge in a graph, there is
exactly one (other) vertex in the graph being adjacent to the given
vertex by the given edge, analogous to usgraedgreu24606. (Contributed by
Alexander van der Vekens, 4-Jan-2018.) (Revised by AV, 9-Jan-2020.)
(New usage is discouraged.) (Proof modification is
discouraged.)